1. Field of the Invention
The present invention relates to a particularly advantageous process for the catalytically supported combustion of carbonaceous materials, including natural gas and methane. In a more specific aspect, this invention relates to a process for catalytically-supported combustion of natural gas or methane using a supported palladium oxide catalyst, without the formation of substantial amounts of nitrogen oxides.
Burning of carbonaceous fuels is associated with formation of air pollutants, among the most troublesome of which are nitrogen oxides (NOx). Nitrogen oxides form whenever air-supported combustion takes place at open flame temperatures. One approach to eliminating nitrogen oxides involves chemically modifying the oxides after their formation. This approach has drawbacks, including the high cost associated with attempting to eliminate 100% of a once-formed pollutant. A more direct method of eliminating nitrogen oxides is to operate the combustion process at a lower temperature so that no formation of nitrogen oxide occurs. Such low temperature combustion can take place in the presence of catalysts, and it is to such a low temperature combustion process that this invention is directed.
In general, conventional adiabatic, thermal combustion systems (e.g., gas turbine engines) operate at such high temperatures in the combustion zone that undesirable nitrogen oxides, especially NO, are formed. A thermal combustion system operates by contacting fuel and air in flammable proportions with an ignition source, e.g., a spark, to ignite the mixture which will then continue to burn. Flammable mixtures of most fuels burn at relatively high temperatures, i.e., about 3,300.degree. F. and above, which inherently results in the formation of substantial amounts of NOx. In the case of gas turbine combustors, the formation of NOx can be reduced by limiting the residence time of the combustion products in the combustion zone. However, due to the large quantities of gases being handled, undesirable quantities of NOx are nonetheless produced.
It has long been realized that little or no NOx is formed in a system which catalytically burns a fuel at relatively low temperatures as compared to uncatalyzed thermal combustion. Typically, such catalytic combustion of natural gas or methane, for example, utilizes a preburner or thermal combustor which employs flame combustion to preheat combustion air to a temperature of 700.degree. C. or higher. Once the catalyst is sufficiently hot to sustain catalysis, the preburner is shut down and all the fuel and air are directed to the catalyst. Preheat is then only due to compressor discharge. Such a catalytic combustor, if operated at temperatures below about 1,300.degree. C.-1,400.degree. C., avoids the nitrogen oxide formation which occurs at the higher temperatures which are characteristic of the flame combustion. A description of such a catalytic combustion process and apparatus is found, for example, in U.S. Pat. No. 3,928,961. See also U.S. Pat. Nos. 4,065,917 and 4,019,316.
Such catalytic combustion as described above which will function effectively at a high space velocity has, however, heretofore been generally regarded as commercially unattractive. A primary reason for this lack of commercial attractiveness has been the absence of an economically competitive method for catalytic combustion of natural gas.
2. Description of Related Art
Catalytically supported combustion processes have been described in the prior art. See, e.g., Pfefferle, U.S. Pat. No. 3,928,961. The use of natural gas or methane in catalytic combustion has been taught in the art, as has the use of a palladium catalyst to promote such combustion/oxidation. See Cohn, U.S. Pat. No. 3,056,646 wherein the use of palladium catalyst to promote methane oxidation is generically disclosed, as is an operable temperature range, 271.degree. C. to 900.degree. C. (see column 2, lines 19-25). Note also that this Patent states "the higher the operating temperature, the shorter will be the catalyst life and the more difficult will be subsequent ignition after catalyst cooling". Other patents directed to the use of platinum group metals as catalysts for methane oxidation at temperatures above 900.degree. C. include U.S. Pat. Nos. 3,928,961; 4,008,037; and 4,065,917. The literature also describes the thermal decomposition of PdO to Pd metal at temperatures of 800.degree. C. in air at atmospheric pressure. See Kirk Othmer Encyclopedia of Chemical Technoloqy, Vol. 18, p. 248 which states that palladium acquires a coating of oxide when heated in air from 350.degree. C. to 790.degree. C. but that above this temperature the oxide decomposes and leaves the bright metal.
The present invention finds particular utility in a process for the start-up of catalytically supported combustion. Prior art references directly related to such start-up are Pfefferle, U.S. Pat. No. 4,019,316 and Pfefferle, U.S. Pat. No. 4,065,917.
C. L. McDaniel et al, "Phase Relations Between Palladium Oxide and the Rare Earth Sesquioxides in Air," Journal of Research of the Natural Bureau of Standards--A. Physics and Chemistry, Vol. 72A, No. 1, January-February, 1968, pages 27-37, describe complexes of PdO and other rare earth oxides. Specifically, the paper describes PdO in combination with each of the following sesquioxides La.sub.2 O.sub.2, Eu.sub.2 O.sub.3, Gd.sub.2 O.sub.3, Dy.sub.2 O.sub.3, Ho.sub.2 O.sub.3, Y.sub.2 O.sub.3, Er.sub.2 O.sub.3, Tm.sub.2 O.sub.3, Yb.sub.2 O.sub.3 and Lu.sub.2 O.sub.3.
A. Kato et al, "Lanthanide B-Alumina Supports For Catalytic Combustion Above 1,000.degree. C.," Successful Design of Catalysts, 1988, Elsevier Science Publishers, pages 27-32, describes the preparation of support materials consisting of lanthanide oxides and alumina for use as combustion catalysts. The preparation comprises preparing a mixed solution of a lanthanide element nitrate (e.g., a nitrate of Y, La, Ce, Pr, Nd, Sm, etc.) and Al.sub.2 (NO.sub.3).sub.3, neutralizing the solution by adding dilute aqueous ammonia to form a precipitate, and washing, drying and calcining the precipitate at 500.degree. C. The powder, with 1% added graphite, was formed into cylindrical tablets and calcined at 700.degree. C. The resultant support was impregnated with a solution of Pd(NO.sub.3).sub.2 to provide 1% by weight Pd, then calcined at 500.degree. C., then at 1,200.degree. C. The article states that the use of La, Pr and Nd as the lanthanide element gave rise to B-alumina (page 28) and that endurance tests on methane combustion performed at 1,200.degree. C. demonstrated that a Pd catalyst supported on lanthanum B-alumina has good durability and resistance to thermal sintering (pages 31 and 32).